1. Field of the Invention
The present invention relates to a wireless base station that uses an adaptive array method to form wireless connections with a plurality of mobile stations using spatial multiplexing.
2. Description of the Related Art
In recent years, an increase in the number of mobile stations such as PHS (Personal Handyphone System) handsets and mobile phones has heightened social requirements for more effective use of communication frequencies. One response to this demand is to perform communication using a spatial multiplexing method.
Such a method involves using highly directive antennas for transmission and reception in a wireless base station, to multiplex a plurality of mobile stations located in different directions on a single frequency channel.
One example of the highly directive antennas that may be used in such a spatial multiplexing method is an adaptive array apparatus. An adaptive array apparatus includes a plurality of positionally fixed antennas. By dynamically adjusting the amplitude and the phase of transmission and reception signals for each antenna, the adaptive array apparatus uses the antennas collectively to form a directional pattern (also known as an array antenna pattern) dynamically each time transmission or reception is performed.
The direction pattern formed by the array antenna apparatus enhances receptivity and transmissivity in the direction of a desired mobile station (also referred to as beam directing or beam steering) as well as reducing receptivity and transmissivity in the direction of other multiplexed mobile stations (also referred to as null directing or nulling out).
The adaptive array apparatus can be easily installed since it is composed of a number of individual fixed antennas, and due to its ability to form a dynamic directional pattern, is particularly suited for use in wireless base stations that need to track the movement of mobile stations. Consequently, adaptive array apparatuses make effective use of frequency resources and are being widely used commercially.
Further details regarding an adaptive array apparatus are contained in Kukan Ryoiki Ni Okeru Tekioshingo Shori To Sono Oyogijyutsu Ronbuntokushu (Adaptive Signal Processing and Applied Techniques in the Spatial Domain: Special Edition) in Denshi Tsushin Gakkai Ronbunshi (The Transactions of the Institute of Electronics, Information, and Communication Engineers (IEICE)) Vol. J75-B-II No. 11, November), so a detailed explanation is omitted here.
When an adaptive array apparatus is used in a wireless base station, it changes the directional pattern to track the movement of each multiplexed mobile station, thereby avoiding interference and maintaining communication performance levels. This operation is conventionally performed as follows.
If a signal from a mobile station to be tracked is received, the adaptive array apparatus controls the amplitude and phase of reception signals for each antenna so as to reduce an error between the reception signal and a predetermined part of the original signal (hereafter referred to as a xe2x80x98reference signalxe2x80x99). The reference signal may correspond, for example, to a UW (unique word) field, if the signal is compliant with the PHS standard. By performing such a control, the adaptive array apparatus directs the beam in the direction of the mobile station to be tracked, and nulls out other multiplexed mobile stations.
A base station using a conventional adaptive array apparatus can avoid interference and maintain communication performance by means of this kind of control.
However, using such a conventional control may result in the following problem. When a further mobile station (hereafter the specified mobile station) is to be spatially-multiplexed, the noise level of the channel to which the specified mobile station has been allocated is measured. Here, if another mobile station is already wireless-connected on the same frequency as the specified mobile station, the noise level will not be kept below a reference-value, and in such a case the channel is frequently judged as having excess noise.
As a result, having made a connection using such a method, the specified mobile station will be able to communicate with the base station without inference from signals emitted by the wireless base station to other mobile stations by having the wireless base station form appropriate directional patterns to null out the specific mobile station, but it will not have the opportunity to make a connection on channels on which the other mobile stations are already communicating.
This problem is likely to occur with more frequency when the wireless base station and the specified mobile station are close together, and the noise level of the directional pattern for other already multiplexed mobile stations cannot be sufficiently reduced.
In order to resolve the above problems, the present invention has as its object the provision of a wireless base station that reduces the influence of carrier waves for already multiplexed mobile stations when the noise level of a channel to be allocated to a specified mobile station is measured, thereby improving the chances of obtaining a connection.
In order to achieve this object, a wireless base station of the present invention uses an adaptive array method to form wireless connections with a plurality of mobile stations using spatial multiplexing. The wireless base station includes a determining unit, a first calculating unit, and a control unit. The determining unit determines a direction in which a specified mobile station that is to be spatially-multiplexed is located. The first calculating unit calculates a first parameter group, used to form a first directional pattern for each already connected mobile station that should be spatially-multiplexed with the specified mobile station. The first directional pattern is such that a null point is formed in the determined direction. The controlling unit performs control, after a link channel allocation has been transmitted to the specified mobile station, so that transmission is performed to each already connected mobile station by forming the corresponding first directional pattern and using reduced transmission power.
This structure reduces the influence of carrier waves for already multiplexed mobile stations, when the noise level of a channel to be allocated to a specified mobile station is measured.
The wireless base station may further include a second calculating unit that calculates a second parameter group used to form a second directional pattern that optimizes signals to and from each already connected mobile station. Here, the control unit performs control to perform transmission for each already connected mobile station by forming the corresponding second directional pattern and returning transmission power to a normal level. The control is performed when a sync burst signal is received from the specified mobile station, or a specified time expires without a sync burst signal being received from the specified mobile station.
This structure enables the wireless base station, once the noise of the specified mobile station is measured, to return the transmission power of signals output to already connected mobile stations back to a normal level, while maintaining the communication performance of these mobile stations.
The wireless base station may further include a detecting unit that detects a signal level of each already connected mobile station. Here, the control unit reduces the transmission power to each already connected mobile station, based on a corresponding detected signal level.
By using this structure, the wireless base station can restrict transmission power to a low level, if transmitting to a distant mobile station that has already been spatially-multiplexed. This reduces the risk of communications breaking down.
The wireless base station may further include a detecting unit for detecting a signal level of each already connected mobile station. Here, the control means reduces the transmission power to each already connected mobile station, based on a corresponding detected signal level.
This structure enables the same effect to be achieved as for the structure above.
A control method of the present invention may be used by a wireless base station that uses an adaptive array method to form wireless connections with a plurality of mobile stations using spatial multiplexing. The control method comprises the following steps. A determining step determines a direction in which a specified mobile station that is to be spatially-multiplexed is located. A first calculating step calculates a first parameter group, used to form a first directional pattern for each already connected mobile station that should be spatially-multiplexed with the specified mobile station, the first directional pattern being such that a null point is formed in the determined direction. A controlling step performs control, after a link channel allocation has been transmitted to the specified mobile station, so that transmission is performed to each already connected mobile station by forming the corresponding first directional pattern and using reduced transmission power.
A wireless base station controlled by this control method enables the same effect to be obtained as for above.